Hydrophobic catalytic materials and method of forming the same

ABSTRACT

A catalytic composition and method of making the same in which a catalytic material has an average pore size distribution sufficiently large to substantially prevent capillary condensation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/430,141 filed May 6, 2003, U.S. Pat. No. 6,685,898; which is adivisional of U.S. application Ser. No. 09/716,035 filed Nov. 17, 2000,U.S. Pat. No. 6,586,359; which is a divisional of U.S. application Ser.No. 09/046,103 filed Mar. 23, 1998, U.S. Pat. No. 6,156,283; all of theforegoing are hereby incorporated by reference into this application.

FIELD OF THE INVENTION

The present invention is directed to a method of treatingpollutant-containing gases in which such gases are contacted with acatalyst composition containing at least one catalytic material whichhas an average pore size and surface area sufficient to prevent or atleast substantially reduce capillary condensation.

BACKGROUND OF THE INVENTION

The present invention is directed to a method of forming catalyticmaterials in such a manner that the catalytic material does notsubstantially undergo capillary condensation. Accordingly, the adverseeffect that water vapor has on catalytic activity of the catalyticmaterial is minimized.

Catalytic materials, especially for removing pollutants from apollutant-containing gas are generally comprised of metals as well asother constituents which actively induce a chemical reaction. Theeffectiveness of a catalytic material depends in part on theavailability of catalytically active sites. The more catalyticallyactive sites available from a given catalyst, the more efficiently thecatalytic material can promote the desired reaction.

Catalytic materials are used to induce the reaction of pollutantscontained within a pollutant-containing gas into harmless by-products.There are numerous pollutants which are found in the atmosphere and/orcontained within gas discharged from industrial facilities or automotivevehicles. Such pollutants include hydrocarbons, carbon monoxide, ozone,sulfur compounds and NO_(x) compounds.

If a potentially catalytically active site is blocked then itsavailability to catalyze the chemical reaction of a pollutant iseliminated or at least substantially reduced. Compounds which blockcatalytically active sites do so by binding to the catalytic material sothat at least a portion of the time the catalytically active site isunavailable for catalyzing a reaction. The stronger the bond between theblocking compound and the catalytically active site, the less efficientthe catalytic material is in inducing a chemical reaction to convertpollutants to harmless by-products.

It is well known that water molecules have an affinity for catalyticmaterials, especially metals. Accordingly, water serves as a blockingcompound which reversibly binds to catalytically active sites. The bondbetween water molecules and catalytically active sites is typically ofmoderate strength so that the water molecules spend only a portion ofthe time bound to the catalytically active site. When the water moleculeis so bound, the particular catalytically active site is incapable ofinducing a chemical reaction to convert pollutants to harmlessby-products.

Catalytic materials including those incorporating precious metals, basemetals and the like are employed in catalytic compositions for thetreatment of pollutant-containing gases such as exhaust gas fromautomotive vehicles. The exhaust gases typically contain moisture orwater vapor and the amount of water vapor will vary depending onclimatic conditions. As previously indicated, the presence of watermolecules can impede the effectiveness of a catalytic material becausewater acts as a blocking compound.

During normal operation of an automotive vehicle, the temperature of theexhaust gas will be several hundred degrees. Under these hightemperature conditions, water molecules are energized due to the inputof thermal energy. Highly energized molecules tend to remain in motion.This high energy level limits the time the water molecules remain boundto catalytically active sites. Accordingly, the presence of water vaporunder high temperature operating conditions does not adversely affectthe efficiency of catalytic materials to the same extent as under lowertemperature operating conditions when water molecules are lessenergized. Under less energized conditions, water molecules tend to bindto catalytically active sites for a greater length of time than underhigh energy conditions (e.g. higher temperatures).

Catalytic materials are generally manufactured with a preference forhigh surface areas so as to enable a greater number of catalytic sitesto catalyze the reaction of pollutants contained within apollutant-containing gas. High surface area catalytic materials can beproduced by employing a pore structure comprised of micropores having anaverage pore size as low as possible, typically less than 5 nanometers(nm). Smaller pores therefore, are characteristic of high surface areacatalytic materials.

It has been observed that catalytic materials having an average poresize of less than about 5 nm, undesirably retain moisture especiallyunder high humidity and low temperature (i.e. low energy) conditions.When water vapor is in contact with such materials, molecules of waterenter the relatively small pores and remain within the pores. Thisphenomenon is known as capillary condensation.

“Capillary condensation” as used herein means that water molecules enterand remain within the micropore structure of the catalytic material.Because the micropores have very small pore sizes (typically less than 5nm), the water molecules become “stuck” in the pores and can be removedonly with some difficulty. The retention of water molecules inmicropores (capillary condensation) reduces the effectiveness ofcatalytic materials because the water molecules block the catalyticallyactive sites as previously described. In particular, the number ofcatalytically active sites available to catalyze the reaction of apollutant is reduced and therefore the efficiency of the catalyticmaterial is impaired.

It would therefore be a significant advance in the art of removingpollutants from a pollutant-containing gas to provide catalyticmaterials in which capillary condensation is prevented or at leastsubstantially minimized. It would be another advance in the art toproduce catalytic materials which can be used in automotive vehicles toremove pollutants from a pollutant-containing gas under high humidityand/or low temperature operating conditions.

SUMMARY OF THE INVENTION

The present invention is generally directed to a method of treating apollutant-containing gas with a catalytic material in which the presenceof water vapor, even under high relative humidity conditions and/or lowtemperature operating conditions, does not substantially adverselyaffect catalyst performance. Catalytic materials which can perform inthis manner are also encompassed by the present invention.

In particular, the present invention is directed to a catalyticcomposition and method of treating a pollutant-containing gas comprisingcontacting the pollutant-containing gas with a catalyst compositioncontaining at least one catalytic material which has an average poresize of at least 5 nm and a surface area sufficiently large to enablethe catalytic material to react with the pollutant in thepollutant-containing gas. As a result capillary condensation is at leastsubstantially prevented whereby there is sufficient accessability of thecatalytically active sites to induce a reaction of the pollutant toproduce harmless by-products.

In a preferred form of the invention, there is provided a method oftreating a pollutant-containing gas even under high humidity and/orreduced temperature conditions in which the catalytic material has anaverage pore size of at least 10 nm. Catalytic materials employed in thepresent method are also the subject of the present invention.

The catalytic material, in addition to having an average pore size of atleast 5 nm distribution as described above, also has a relatively largesurface area, typically at least 100 m²/g. Preferably, the catalyticmaterial has a relatively high total pore volume, typically at least 0.9cm³/g.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of embodiments of the inventionand are not intended to limit the invention as encompassed by the claimsforming part of the application.

FIG. 1 is a graph showing the amount of water adsorbed by two catalystsas a function of relative humidity;

FIG. 2 is a graph showing capillary condensation as a function ofaverage pore size for a given catalytic material; and

FIG. 3 is a graph showing the efficiency of ozone conversion as afunction of relative humidity.

DETAILED DESCRIPTION OF THE INVENTION

The catalytic materials of the present invention have an average poresize and a surface area sufficient to prevent or at least substantiallyreduce capillary condensation. In a preferred form of the invention, thetotal pore volume of the catalytic material is selected to furtherminimize capillary condensation. As previously explained, capillarycondensation occurs when water molecules enter micropores of a catalyticmaterial and become retained therein because the small size of the poresmakes it difficult for the water to be removed in the absence of raisingthe energy level of the water molecules such as occurs at elevatedtemperatures (e.g. temperatures of at least about 100° C.). As usedherein the term “average pore size” shall mean the average pore diameterof the pores of the catalytic material.

It has been observed that condensation of water occurs in the pores ofcatalytic materials having an average pore size of less than 5 nm,especially at a relative humidity of at least about 50%. The relativehumidity is based on the partial pressure of the water in the air andthe saturation vapor pressure at the catalyst operating temperature. Thefiling of pores with water molecules via capillary condensation isgoverned by the formula${{{RT}\quad{\ln\left( \frac{P}{P} \right)}} = \frac{{- {VY}}\quad{\cos(\theta)}}{D}},$where R is the gas constant (8.31 Joules/mole/K), T is the operatingtemperature of catalyst system, ln is the natural logarithm, P is thepartial pressure of water vapor, P₀ is the saturation vapor pressure ofwater at the operating temperature T, V is the molar volume of water (18cm³/mole), γ is the surface tension of water (72.6 dynes/cm), θ is thecontact angle and D is the capillary diameter (cm). The contact angle isthe angle formed between the liquid surface and a solid surface. Forhydrophilic surfaces, the contact angle is generally between about 0 and90 degrees. For hydrophobic surfaces, the contact angle is generallybetween about 90 and 180 degrees. The contact angle for metal oxidesurfaces (e.g. alumina) is typically less than 90°.

Applicants have discovered that when a catalytic material is providedwith an average pore size of at least 5 nm and a surface area of atleast 100 m²/g capillary condensation is prevented or at leastsubstantially reduced.

Referring to FIG. 1 there is shown a graph depicting the relationshipbetween water adsorption and relative humidity for two differentmanganese oxide based catalytic materials. The first of the catalyticmaterials is Carulite® 200 produced by Carus Chemicals, Inc. and thesecond is HSA (a high surface area) MnO₂ produced by Chemetals, Inc. Asshown in FIG. 1, water adsorption for each of the catalytic materials isrelatively low until the relative humidity reaches about 50%. Wateradsorption below a relative humidity of about 50% is due principally tomulti-layer adsorption. Multilayer adsorption is the formation ofmultiple thin layers of moisture which does not substantially preventaccess of the pollutant-containing gas to the catalytic materialcontained within the pores. At a relative humidity of about 50%, theamount of water adsorbed increases significantly due to capillarycondensation.

In accordance with the present invention, the average pore size and therelative humidity of the atmosphere impact on whether or not capillarycondensation occurs.

As shown in FIG. 2 and in accordance with the present invention, as theaverage pore size increases, the relative humidity necessary to inducecapillary condensation significantly increases. Capillary condensationis initiated at low pore sizes (an average pore size of less than 5 nm)at low relative humidity conditions (i.e. ≦50%). Thus increasing theaverage pore size to at least 5 nm prevents or substantially reducescapillary condensation to relative humidities of up to about 50% whenthe catalytic material has a surface area of at least 100 m²/g.Increasing the average pore size to a preferred range of at least 10 nmand more preferably from about 15 to 50 nm substantially eliminatescapillary condensation to relative humidities up to about 75%.

The effect of relative humidity on the ability of a catalyst to convertozone to harmless byproducts is shown in FIG. 3. These results wereobtained by passing 1.53 L/min of air containing 5 ppm of ozone through50 mg of high surface area α-MnO₂ at 25-27° C. As shown in FIG. 3, about65-68% conversion of ozone was achieved at a relative humidity of fromabout 50 to 60%. However, as the relative humidity increased, especiallyabove 60% (e.g. 90%) there was a noticeable decline in the ozoneconversion rate to 19%. The significant decline in conversion rate isdue at least in large part to the presence of water in vicinity of thecatalytically active sites due to capillary condensation.

The catalytic materials which can be employed in the present inventioncan vary widely but generally include platinum group metals, basemetals, alkaline earth metals, rare earth metals and transition metals.

The platinum group metals include platinum, palladium, iridium, andrhodium. The base metals include manganese, copper, nickel, cobalt,silver and gold. The alkaline earth metals include beryllium, magnesium,calcium, strontium, barium, and radium. The rare earth metals includecesium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,and lutetium. Early transition metals include scandium, yttrium,lanthanum, tympanum, zirconium, and hafnium.

Examples of such catalytic materials are disclosed in U.S. Pat. No.5,139,992, U.S. Pat. No. 5,128,306, U.S. Pat. No. 5,057,483, U.S. Pat.No. 5,024,981, U.S. Pat. No. 5,254,519, and U.S. Pat. No. 5,212,142,each of which is incorporated herein by reference.

The most preferred catalytic materials for use in the present inventionare those which contain manganese and particularly those which containmanganese dioxide as explained in detail hereinafter. Such catalyticmaterials are especially suitable for treating ozone.

Ozone treating catalyst compositions comprise manganese compoundsincluding manganese dioxide, non stoichiometric manganese dioxide (e.g.,XMnO_((1.52.0))), and/or XMn₂O₃ wherein X is a metal ion, preferably analkali metal or alkaline earth metal (e.g. sodium, potassium andbarium). Variable amounts of water (H₂O, OH⁻) can be incorporated in thestructure as well. Preferred manganese dioxides, which are nominallyreferred to as MnO₂ have a chemical formula wherein the molar ratio ofmanganese to oxide is about from 1.5 to 2.0. Up to 100 percent by weightof manganese dioxide MnO₂ can be used in catalyst compositions to treatozone. Alternative compositions which are available comprise manganesedioxide and compounds such as copper oxide alone or copper oxide andalumina. In accordance with the present invention, the most dramaticimprovement in catalytic efficiency with higher pore size distributionis seen with catalyst containing manganese dioxide alone.

Useful and preferred manganese dioxides are alpha-manganese dioxidesnominally having a molar ratio of manganese to oxygen of from 1 to 2.Useful alpha manganese dioxides are disclosed in U.S. Pat. No. 5,340,562to O'Young, et al.; also in O'Young, Hydrothermal Synthesis of ManganeseOxides with Tunnel Structures presented at the Symposium on Advances inZeolites and Pillared Clay Structures presented before the Division ofPetroleum Chemistry, Inc. American Chemical Society New York CityMeeting, Aug. 25-30, 1991 beginning at page 342; and in McKenzie, theSynthesis of Birnessite, Cryptomelane, and Some Other Oxides andHydroxides of Manganese, Mineralogical Magazine, December 1971, Vol. 38,pp. 493-502. For the purposes of the present invention, the preferredalpha-manganese dioxide is selected from hollandite (BaMn₈O₁₆.xH₂O),cryptomelane (KMn₈O₁₆.xH₂O), manjiroite (NaMn₈O₁₆.xH₂O) or coronadite(PbMn₈O₁₆.xH₂O).

The manganese dioxides useful in the present invention preferably have asurface area as high as possible while maintaining a pore sizedistribution of at least 10 nm. A preferred surface area is at least 100m²/g.

The composition preferably comprises a binder as of the type describedbelow with preferred binders being polymeric binders. The compositioncan further comprise precious metal components with precious metalcomponents being the oxides of precious metal, including the oxides ofplatinum group metals and oxides of palladium or platinum also referredto as palladium black or platinum black. The amount of palladium orplatinum black can range from 0 to 25%, with useful amounts being inranges of from about 1 to 25 and 5 to 15% by weight based on the weightof the manganese component and the precious metal component.

It has been found that the use of compositions comprising thecryptomelane form of alpha manganese oxide, which also contain apolymeric binder can result in greater than 50%, preferably greater than60% and typically from 75-85% conversion of ozone in a concentrationrange of up to 400 parts per billion (ppb).

The preferred cryptomelane manganese dioxide has a crystallite sizeranging from 2 to 10 nm and preferably less than 5 nm. It can becalcined at a temperature range of from 250° C. to 550° C. andpreferably below 500° C. and greater than 300° C. for at least 1.5 hoursand preferably at least 2 hours up to about 6 hours.

The preferred cryptomelane can be made in accordance with methodsdescribed and incorporated into U.S. patent application Ser. No.08/589,182 filed Jan. 19, 1996, incorporated herein by reference. Thecryptomelane can be made by reacting a manganese salt including saltsselected from the group consisting MnCl₂, Mn(NO₃)₂, MnSO₄ andMn(CH₃COO)₂ with a permanganate compound. Cryptomelane is made usingpotassium permanganate; hollandite is made using barium permanganate;coronadite is made using lead permanganate; and manjiroite is made usingsodium permanganate. It is recognized that the alpha-manganese dioxideuseful in the present invention can contain one or more of hollandite,cryptomelane, manjiroite or coronadite compounds. Even when makingcryptomelane minor amounts of other metal ions such as sodium may bepresent. Useful methods to form the alpha-manganese dioxide aredescribed in the above references which are incorporated herein byreference.

The preferred alpha-manganese dioxide for use in accordance with thepresent invention is cryptomelane. The preferred cryptomelane is “clean”or substantially free of inorganic anions, particularly on the surface.Such anions could include chlorides, sulfates and nitrates which areintroduced during the method to form cryptomelane. An alternate methodto make the clean cryptomelane is to react a manganese carboxylate,preferably manganese acetate, with potassium permanganate.

It is believed that the carboxylates are burned off during thecalcination process. However, inorganic anions remain on the surfaceeven during calcination. The inorganic anions such as sulfates can bewashed away with the aqueous solution or a slightly acidic aqueoussolution. Preferably the alpha manganese dioxide is a “clean” alphamanganese dioxide. The cryptomelane can be washed at from about 60° C.to 100° C. for about one-half hour to remove a significant amount ofsulfate anions. The nitrate anions may be removed in a similar manner.The “clean” alpha manganese dioxide is characterized as having an IRspectrum as disclosed in U.S. patent application Ser. No. 08/589,182filed Jan. 19, 1996.

A preferred method of making cryptomelane useful in the presentinvention comprises mixing an aqueous acidic manganese salt solutionwith a potassium permanganate solution. The acidic manganese saltsolution preferably has a pH of from 0.5 to 3.0 and can be made acidicusing any common acid, preferably acetic acid at a concentration of from0.5 to 5.0 normal and more preferably from 1.0 to 2.0 normal. Themixture forms a slurry which is stirred at a temperature range of from50° C. to 110° C. The slurry is filtered and the filtrate is dried at atemperature range of from 75° C. to 200° C. The resulting cryptomelanecrystals have a surface area of typically in the range of at least 100m²/g.

Catalytic materials with an average pore size of at least 5 nm inaccordance with the present invention can be made, for example, by heattreating the material after crystallization. The post-crystallizationmaterial can be heated to temperatures sufficient to increase theaverage pore size to at least 5 nm. In most cases, thepost-crystallization heat-treating temperature is in the range of fromabout 300 to 500° C.

Other useful compositions comprise manganese dioxide and optionallycopper oxide and alumina and at least one precious metal component suchas a platinum group metal supported on the manganese dioxide and wherepresent copper oxide and alumina. Useful compositions contain up to 100,from 40 to 80 and preferably 50 to 70 weight percent manganese dioxide10 to 60 and typically 30 to 50 percent copper oxide. Usefulcompositions include hopcalite (supplied by, for example, Mine SafetyApplications, Inc.) which is about 60 percent manganese dioxide andabout 40 percent copper oxide: and Carulite® 200 (sold by Carus ChemicalCo.) which is reported to have 60 to 75 weight percent manganesedioxide, 11 to 14 percent copper oxide and 15 to 16 percent aluminumoxide. The surface area of Carulite® 200 is reported to be about 180m²/g. Calcining at 450° C. reduces the surface area of the Carulite® byabout fifty percent (50%) without significantly affecting activity. Itis preferred to calcine manganese compounds at from 300° C. to 500° C.and more preferably 350° C. to 450° C. Calcining at 550° C. causes agreat loss of surface area and ozone treatment activity. Calcining theCarulite® after ball milling with acetic acid and coating on a substratecan improve adhesion of the coating to a substrate.

Other compositions to treat ozone can comprise a manganese dioxidecomponent and precious metal components such as platinum group metalcomponents. While both components are catalytically active, themanganese dioxide can also support the precious metal component. Theplatinum group metal component preferably is a palladium and/or platinumcomponent. The amount of platinum group metal compound preferably rangesfrom about 0.1 to about 10 weight percent (based on the weight of theplatinum group metal) of the composition. Preferably, where platinum ispresent it is present in amounts of from 0.1 to 5 weight percent, withuseful and preferred amounts on pollutant treating catalyst volume,based on the volume of the supporting article, ranging from about 0.5 toabout 70 g/ft³. The amount of the palladium component preferably rangesfrom about 2 to about 10 weight percent of the composition, with usefuland preferred amounts on pollutant treating catalyst volume ranging fromabout 10 to about 250 g/ft³.

Various useful and preferred pollutant treating catalyst compositions,especially those containing a catalytically active component such as aprecious metal catalytic component, can comprise a suitable supportmaterial such as a refractory oxide support. The preferred refractoryoxide can be selected from the group consisting of silica, alumina,titania, ceria, zirconia and chromia, and mixtures thereof. Morepreferably, the support is at least one activated, high surface areacompound selected from the group consisting of alumina, silica, titania,silica-alumina, silica-zirconia, alumina silicates, alumina zirconia,alumina-chromia and alumina-ceria. The refractory oxide can be insuitable form including bulk particulate form typically having particlesizes ranging from about 0.1 to about 100 and preferably 1 to 10 μm orin sol form also having a particle size ranging from about 1 to about 50and preferably about 1 to about 10 nm. A preferred titania sol supportcomprises titania having a particle size ranging from about 1 to about10, and typically from about 2 to 5 nm.

Also useful as a preferred support is a coprecipitate of a manganeseoxide and zirconia. This composition can be made as recited in U.S. Pat.No. 5,283,041, incorporated herein by reference. The coprecipitatedsupport material preferably comprises in a ratio based on the weight ofmanganese and zirconium metals from about 5:95 to 95:5; preferably fromabout 10:90 to 75:25; more preferably from about 10:90 to 50:50; andmost preferably from about 15:85 to 50:50. A useful and preferredembodiment comprises a Mn:Zr weight ratio of about 20:80. U.S. Pat. No.5,283,041 describes a preferred method to make a coprecipitate of amanganese oxide component and a zirconia component. A zirconia oxide andmanganese oxide material may be prepared by mixing aqueous solutions ofsuitable zirconium oxide precursors such as zirconium oxynitrate,zirconium acetate, zirconium oxychloride, or zirconium oxysulfate and asuitable manganese oxide precursor such as manganese nitrate, manganeseacetate, manganese dichloride or manganese dibromide, adding asufficient amount of a base such as ammonium hydroxide to obtain a pH offrom about 8 to 9, filtering the resulting precipitate, washing withwater, and drying at a temperature of from about 450° to 500° C.

A useful support for a catalyst to treat ozone is selected from arefractory oxide support, preferably alumina and silica-alumina with amore preferred support being a silica-alumina support comprising fromabout 1% to 10% by weight of silica and from about 90% to 99% by weightof alumina.

The average pore size of the catalytic material including the preferredmaterials described above is at least 5 nm, preferably at least 10 nm,more preferably from about 15 to 50 nm. At this average pore size, anywater molecules which enter the pores are readily disengaged fromcatalytic sites without the imposition of excessive energy such asthermal energy. Accordingly, the method of the present invention isparticularly suited to the catalytic conversion of apollutant-containing gas (e.g. exhaust gas) at reduced operatingtemperatures. The method of the present invention is particularly suitedto the efficient and effective conversion of pollutants to harmlessby-products when an engine of an automotive vehicle is under startupconditions (i.e. generally less than 45° C.). The catalytic materials ofthe present invention are particularly suited for the conversion ofcarbon monoxide and ozone.

EXAMPLE 1

61 grams of a powdered catalytic material containing noncrystallineKMn₈O₁₆ prepared by methods described in “Microstructural Study ofHollandite-type MnO₂ Nano-fibers” M. Benaissa et al. App. PhysicsLetters, Vol. 70, No. 16, pp.2120-2122 (1997) and “Nickel Hydroxide andother Nanophase Cathode Materials For Rechargeable Batteries” D. E.Reisner et al. J. Power Sources Vol. 65, No. 1-2, pp. 231-233 (1997) wastreated in the following manner. The material is comprised of primarycrystallites having a fibrous shape with aspect ratios of about 10:1(i.e. 10-100 nm wide×100-1,000 nm long). The fibrous crystallites formloosely compacted subspherical agglomerates resembling nests up to about10 μm across. The bulk of density of the material is about 0.3 to 0.6g/cm³. The powder was pressed, granulated and sized so that theresulting material had an average pore size of 32.2 nm, a pore volume(the total volume divided by the average pore size) of 0.98 cm³/g and asurface area of 122 m²/g. Equal volumes (0.13 cm³) of the powder wereloaded into glass tubes and secured into a bed with glass wool. Thesamples were run on a temperature and humidity controlled flow reactorequipped with an ozone generator and UV ozone analyzer.

A gas containing 5 ppm of ozone in air was passed through the sample bedat a space velocity of 150,000 hr⁻¹ at 45° C. with a dew point of 17° C.for 2 hours. The instantaneous conversion of ozone to oxygen at the endof two hours is shown in Table 1.

TABLE 1 AVG. OZONE SURFACE PORE PORE CONVERSION AREA^(a), VOLUME^(b),DIAMETER % SAMPLE m²/g cm³/g nm MASS EXAM- 122 0.98 32.2 99  61 mg PLE 1COM- 44 0.21 18.6 99  91 mg PAR- ATIVE EXAM- PLE 1 COM- 84 0.20 9.4 72135 mg PAR- (1.3 hr) ATIVE EXAM- PLE 2 ^(a)BET surface area measured byN₂ adsorption after drying sample at 250° C. ^(b)Total pore volume andaverage pore diameter determined by N₂ adsorption.

As shown in Table 1, the % conversion of ozone for Example 1 was 99%.This example employed an average pore diameter and surface area whichminimized capillary condensation. The % conversion was achieved with alow mass of catalytic material.

COMPARATIVE EXAMPLE 1

91 grams of a starting material containing activated MnO₂ obtained fromJohnson Matthey Alfa Aesar (Technical Grade Stock No. 14340) wascomprised of nearly equant or subspherical crystallites with diametersof from about 50 to 100 nm. The crystallites form dense aggregates andagglomerates up to about 20 μm across. The bulk density of this materialis from about 1.1 to 1.3 g/m². The final catalytic material was obtainedby heating the starting material to 450° C. for 2 hours to obtain anaverage pore size distribution of 18.6 nm.

As shown in Table 1 the % conversion of ozone initiated by the catalyticmaterial was 99%. However, a larger mass of catalytic material wasneeded to achieve high conversion rates due to the relatively smallsurface area (44 m²/g) and pore volume (0.21 cm³/g) as compared to thecatalytic material of Example 1.

COMPARATIVE EXAMPLE 2

The same catalytic material employed in Comparative Example 2 was usedexcept that the step of heating to 450° C. was omitted. The resultingmaterial was comprised of nearly equant or subspherical crystallites onthe order of about 5 to 25 nm across that form subspherical aggregatesand agglomerates up to 20 μm across, with an average pore sizedistribution of 9.4 nm. The bulk density of the material is between 0.9and 1.1 g/cm³.

The catalytic material was treated in the same manner as in Example 1and Comparative Example 2 except that the reaction was terminated after1.3 hours because the conversion rates were decreasing. The results ofthis comparative example showing the effectiveness of converting ozoneto oxygen is shown in Table 1. The conversion rate for this comparativeExample was only 72% despite using more than twice the amount of thecatalytic material.

As shown in Table 1, the highest conversion rate with the lowest mass ofmaterial is achieved when the catalytic material (Example 1) has anaverage pore size equal to or exceeding about 5.0 nm, a pore volumeequal to or exceeding about 0.9 cm³/g and a surface area equal to orexceeding 100 m²/g.

As further shown in Table 1, the comparative examples showedsignificantly lower conversion rates for even greater masses ofmaterial.

1. A catalytic composition, the catalytic composition comprising atleast one catalytic material comprising manganese and having an averagepore size of at least 5 nm, a surface area of at least 100 m²/g and apore volume of at least 0.9 cm³/g.